[quote="Kaizmom, post:1, topic:186071"]
I have noticed that during some parts of the mass, some of the congregation make a fist with their right hand and tap their chest with it (over their hearts).
During the Confiteor ("I confess to almighty God..."), one form of the Penitential Act, we strike our breast at specific words. Here's an excerpt on why we do this and what it means, from my Praying the Mass: The Prayers of the People:The first half of the Confiteor ends with an admission of personal guilt for our sins. As we say these words, we strike our breast three times in a sign of penitence:
mea culpa, mea culpa, mea máxima culpa.
through my fault, through my fault, through my most grievous fault; [Sir. 20:2b]
The repetition of this admission of guilt adds to its severity. We do not say “The devil made me do it, the devil made me do it, you can bet the devil made me do it,” but accuse only ourselves for our sins. We beat our breast with a closed fist, like the tax collector who prayed from his heart, “God, be merciful to me a sinner!” (Luke 18:13) Concerning the gravity of these words and this gesture, Cardinal Ratzinger wrote:[INDENT]We point not at someone else but at ourselves as the guilty party, [which] remains a meaningful gesture of prayer. … When we say mea culpa (through my fault), we turn, so to speak, to ourselves, to our own front door, and thus we are able rightly to ask forgiveness of God, the saints, and the people gathered around us, whom we have wronged. (The Spirit of the Liturgy, p. 207)Rev. Romano Guardini explained that the meaning of this gesture of contrition depends upon it being done properly:To brush one’s clothes with the tips of one’s fingers is not to strike the breast. We should beat upon our breasts with our closed fists. … It is an honest blow, not an elegant gesture. To strike the breast is to beat against the gates of our inner world in order to shatter them. This is its significance. … “Repent, do penance.” It is the voice of God. Striking the breast is the visible sign that we hear that summons. … Let it wake us up, and make us see, and turn to God. (Sacred Signs)The Douay Catechism (from 1649), a question-and-answer catechism on the doctrines of the Church, included a chapter expounding the essence and ceremonies of the Mass. It explains that the reason for striking the heart is “to teach the people to return into the heart” because it “signifies that all sin is from the heart, and ought to be discharged from the heart, with hearty sorrow.” (p. 125)
[RIGHT]Praying the Mass: The Prayers of the People, pp. 45-46[/RIGHT][/INDENT]
In the Ordinary Form of the Mass, this is the only place the rubrics call for us to strike our breast. In the Extraordinary Form, this also happens when making the response "Lord, I am not worthy..." right before the Communion.procession. Here again from my book is an explanation about that:
The Extraordinary Form of the Mass includes a three-fold striking of the breast while this response Domine, non sum dignus..., that is, "Lord, I am not worthy..."] is said three times. Although it is not presently prescribed in the Ordinary Form of the Mass, Pope Benedict, writing before his election to the papacy, considered that at this moment, “we look upon him who is the Shepherd and for us became the Lamb and, as Lamb, bore our iniquities” and that “it is only right and proper that we should strike our breasts and remind ourselves, even physically, that our iniquities lay on his shoulders, that ‘with his stripes we are healed.’” (The Spirit of the Liturgy, p. 207) Rev. Romano Guardini also wrote of this gesture that, when done before Communion, “it is a summons to repentance and to the self-inflicted punishment of a contrite heart.” (Sacred Signs) This gesture may be done as a personal devotion, but it is possible that in the future this pious tradition will be universally re-incorporated into the Roman liturgy.
[RIGHT]Praying the Mass: The Prayers of the People, pp. 146-147[/RIGHT]
As for when to do it, it is up to you if you would like to make it when the Host and Chalice are elevated following their consecration in the Eucharistic Prayer. I would expect people do it as a reminder that Jesus underwent great pain and suffering because of our sins, and striking the breast reminds them of that great Passion of our Lord and stirs in them awe and reverence and gratitude.